Methods and materials for expression of a recombinant protein

ABSTRACT

Recombinant expression vectors are provided comprising a 3′ UTR of a light chain and an Epstein-Barr virus origin of replication. Also provided are host cells comprising such vectors and methods of producing recombinant protein with such vectors. Additional methods of producing a recombinant protein involve contacting cells with a first and second vector, each of which encode a different polypeptide chain, and wherein the second vector is present in an amount which is about 1.5 to 2.5 times as much as that of the first vector. Cells also can be transfected with a recombinant transient expression vector encoding a protein and are cultured in a medium in a membrane-enhanced culturing vessel to produce recombinant protein.

FIELD OF THE INVENTION

This invention pertains to methods of producing a recombinant protein and recombinant expression vectors and host cells for use therein.

BACKGROUND OF THE INVENTION

Large-scale transient expression of recombinant proteins has been an area of rapid development in the past several years as an alternative or precursor to stable cell line development to generate multi-milligram quantities of protein (Wurm et al., Curr. Opn. Biotech. 10: 156-159 (1999)). Human embryonic kidney (HEK293) cells are one of the most widely used cell lines for transient expression and have been successfully adapted to suspension-growth to help facilitate culture scale-up. Recent reports have successfully demonstrated the usage of transiently expressing suspension-adapted HEK293 cells in 1-3 L cultures to generate recombinant proteins including soluble polypeptides, transmembrane proteins, and human antibodies (Durocher et al., Nucleic Acids Res. 30: 1-9 (2002); Meissner et al., Biotechnol. Bioeng. 75: 197-203 (2000); and Cote et al., Biotechnol. Bioeng. 59: 567-575 (1998)).

In particular, Durocher et al. has shown that HEK293E cells expressing the Epstein-Barr virus (EBV) nuclear antigen-1 protein (EBNA1) were able to routinely generate >10 mg/L of a number of different recombinant proteins using the cationic polymer transfection reagent, polyethyleneimine (PEI) (Boussif et al., Proc. Natl. Acad. Sci. 92: 7297-7301 (1995); and Mislick et al., Proc. Natl. Acad. Sci. 93: 12349-12354 (1996)).

Despite these advances, there is still a need in the art for improved expression systems including optimized transient transfection systems for time- and cost-efficient production of recombinant proteins. The invention provides such optimized methods of producing recombinant proteins. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.

BRIEF SUMMARY OF THE INVENTION

The invention provides recombinant expression vectors useful in methods of producing a recombinant protein. One of the inventive recombinant expression vectors comprises a 3′ untranslated region (UTR) of a light chain gene. Another recombinant expression vector provided herein comprises a 3′ UTR and an Epstein-Barr virus origin of replication. Host cells comprising any of the inventive recombinant expression vectors are also provided herein.

The invention further provides methods of producing a recombinant protein. In a first method, the recombinant protein is a heterodimeric or heteromultimeric protein, either of which comprises a first polypeptide chain and a second polypeptide chain, wherein the first polypeptide chain is different from the second polypeptide chain. The method comprises contacting cells in a medium with a first vector and a second vector, wherein the first vector encodes the first polypeptide chain and the second vector encodes the second polypeptide chain, and the second vector is present in the medium in an amount which is about 1.5 to about 2.5 times as much as the amount of the first vector.

In a second method of producing a recombinant protein, the method comprises culturing cells, which have been contacted with a recombinant transient expression vector encoding the protein, in a medium in a membrane-enhanced culturing vessel, whereupon a recombinant protein is produced. Alternatively, the second method comprises culturing cells, which have been contacted with a recombinant transient expression vector encoding the protein, in a medium in a Fembach flask.

In a third method, the recombinant protein is produced upon contacting cells with at least one of the inventive recombinant expression vectors described herein. In a fourth method, the recombinant protein is produced upon culturing host cells comprising any of the inventive recombinant expression vectors described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an illustration of the pMXT recombinant expression vector without any recombinant protein coding sequences. FIG. 1B is an illustration of the pMXT vector encoding a human γ₂ heavy chain, while FIG. 1C is an illustration of the pMXT vector encoding a human κ light chain. The following abbreviations are used in FIGS. 1A-1C: Ap, ampicillin resistance marker; CMV promoter, cytomegalovirus promoter; MCS, multiple cloning sequence; 5′ UT intron, 5′ untranslated region intron; SP, signal peptide; V, variable region; C, constant region; LC 3′ UT, light chain 3′ untranslated region, OriP, Epstein-Barr virus origin of replication; pUC19ori, origin of replication from the pUC19 plasmid.

FIGS. 2A-2I are flow cytometry data graphs depicting the levels of fluorescence of green fluorescence protein (GFP) and propidium iodide (PI) under differing transfection conditions, specifically differing DNA and polyethyleneimine (PEI) concentrations. In FIG. 2A, cells were transfected with 1 μg/ml DNA and 1 μg/ml PEI. In FIG. 2B, cells were transfected with 2 μg/ml DNA and 2 μg/ml PEI. In FIG. 2C, cells were transfected with 5 μg/ml DNA and 5 μg/ml PEI. In FIG. 2D, cells were transfected with 1 μg/ml DNA and 2 μg/ml PEI. In FIG. 2E, cells were transfected with 2 μg/ml DNA and 4 μg/ml PEI. In FIG. 2F, cells were transfected with 5 μg/ml DNA and 10 μg/ml PEI. In FIG. 2G, cells were transfected with 1 μg/ml DNA and 5 μg/ml PEI. In FIG. 2H, cells were transfected with 2 μg/ml DNA and 10 μg/ml PEI. In FIG. 2I, cells were transfected with 5 μg/ml DNA and 25 μg/ml PEI.

FIG. 3 is a graph showing the % cell viability (X) and % GFP positive 293E cells (▪) that were adapted to suspension growth in different serum-free media and optimized for transfection. A control set of 293E cells were grown in DMEM.

FIG. 4 is a graph showing the antibody production by cells which were co-transfected with different heavy chain (HC):light chain (LC) ratios of different vector types.

FIG. 5A is a graph showing antibody production (●) and cell viability (X) of transiently transfected 293E cells in Integra flasks as a function of time post-transfection. FIG. 5B is a graph showing the production of antibodies in shake flasks vs. Integra flasks by transfected cell cultures at day 7 post-transfection.

FIG. 6A is a graph of the percentage of viable cells transfected with DNA encoding Ab#1 as a function of time post-transfection. FIG. 6B is a graph of the percentage of viable cells transfected with DNA encoding Ab#2 as a function of time post-transfection. In both FIGS. 6A and 6B, ▪ is I-50; ▴ is I-100; ● is I-200; ♦ is I-400; and X is E-200.

FIG. 7A is a graph of the number of viable cells transfected with DNA encoding Ab#1 as a function of time post-transfection. FIG. 7B is a graph of the number of viable cells transfected with DNA encoding Ab#2 as a function of time post-transfection. In both FIGS. 7A and 7B, ▪ is I-50; ▴ is I-100; ● is I-200; ♦ is I-400; and X is E-200.

FIG. 8A is a graph of the concentration of antibody produced by cells transfected with DNA encoding Ab#1 as a function of time post transfection. FIG. 8B is a graph of the concentration of antibody produced by cells transfected with DNA encoding Ab#2 as a function of time post-transfection. In both FIGS. 8A and 8B, ▪ is I-50; ▴ is I-100; ● is I-200; ♦ is I-400; and X is E-200.

FIG. 9A is a graph of the total antibody produced by cells tranfected with DNA encoding Ab#1 as a function of time post-transfection. FIG. 9B is a graph of the total yield of antibody produced by cells transfected with DNA encoding Ab#2 as a function of time post-transfection.

FIG. 10 shows SEQ ID NO: 1, which is the nucleotide sequence of pMXT5 (FIG. 1A) without any coding sequences. Restriction enzyme sites are labeled with the name of the enzyme above the position of the site. CMV promoter comprises nucleotides 1-1037; 5′ UTR intron comprises nucleotides 888-974; MCS comprises nucleotides 1038-1061; LC 3′ UT comprises nucleotides 1062-2560; OriP comprises nucleotides 2561-4550; pUC19 ori comprises nucleotides 4551-5220; and Ap comprises nucleotides 5221-6380.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides recombinant expression vectors useful in methods of producing a recombinant protein. One of the inventive recombinant expression vectors comprises a 3′ untranslated region (UTR) of a light chain gene. Another recombinant expression vector provided herein comprises a 3′ UTR and an Epstein-Barr virus origin of replication (oriP). Inventive recombinant expression vectors optionally comprise a pUC19 origin of replication (pUC19ori).

For purposes herein, the term “recombinant expression vector” means a genetically-modified oligonucleotide (i.e., polynucleotide) construct that permits the production of a protein within a cell, when the construct comprises a nucleotide sequence encoding the protein, and the construct is contacted with the cell under conditions sufficient to have the protein expressed within the cell. As the expression vector is recombinant, the vector of the invention is not naturally-occurring as a whole. However, parts of the vectors can be naturally-occurring.

The recombinant expression vector can comprise any type of nucleotides, including, but not limited to DNA and RNA, which can be single-stranded or double-stranded, which can be synthesized or obtained in part from natural sources, and which can contain natural or non-natural or altered nucleotides. Examples of non-natural or altered nucleotides that can be used to generate the recombinant expression vectors include, but are not limited to, 5-fluorouracil, 5-bromouracil, 5-chlorouracil, 5-iodouracil, hypoxanthine, xanthine, 4-acetylcytosine, 5-(carboxyhydroxymethyl) uracil, 5-carboxymethylaminomethyl-2-thiouridine, 5-carboxymethylaminomethyluracil, dihydrouracil, beta-D-galactosylqueosine, inosine, N⁶-isopentenyladenine, 1-methylguanine, 1-methylinosine, 2,2-dimethylguanine, 2-methyladenine, 2-methylguanine, 3-methylcytosine, 5-methylcytosine, N⁶-adenine, 7-methylguanine, 5-methylaminomethyluracil, 5-methoxyaminomethyl-2-thiouracil, beta-D-mannosylqueosine, 5′-methoxycarboxymethyluracil, 5-methoxyuracil, 2-methylthio-N⁶-isopentenyladenine, uracil-5-oxyacetic acid (v), wybutoxosine, pseudouracil, queosine, 2-thiocytosine, 5-methyl-2-thiouracil, 2-thiouracil, 4-thiouracil, 5-methyluracil, uracil-5-oxyacetic acid methylester, 3-(3-amino-3-N-2-carboxypropyl) uracil, (acp3) w, and 2,6-diaminopurine.

The recombinant expression vector can comprise naturally-occurring or non-naturally-occuring internucleotide linkages, or both types of linkages, such as phosphoroamidate linkages or phosphorothioate linkages, instead of the phosphodiester linkages found between the nucleotides of an unmodified oligonucleotide. Preferably, the non-naturally occurring or altered nucleotides or internucleotide linkages do not hinder in any way the transcription or replication of the vector.

The recombinant expression vector can be any suitable recombinant expression vector, and can be used to transform or transfect any suitable host. For example, one of ordinary skill in the art appreciates that transformation or transfection is a process by which, for example, exogenous nucleic acids such as DNA are introduced into cells wherein the transformation or transfection process involves contacting the cells with the exogenous nucleic acids such as the recombinant expression vector as described herein. Suitable vectors include those designed for propagation and expansion or for expression or both, such as plasmids and viruses. The vector can be selected from the group consisting of the pUC series (Fermentas Life Sciences), the pBluescript series (Stratagene, LaJolla, Calif.), the pET series (Novagen, Madison, Wis.), the pGEX series (Pharmacia Biotech, Uppsala, Sweden), and the pEX series (Clontech, Palo Alto, Calif.). Bacteriophage vectors, such as λGT10, λGT11, λZapII (Stratagene), λEMBL4, and λNML149, also can be used. Examples of plant expression vectors include pBI01, pBI101.2, pBI101.3, pBI121, and pBIN19 (Clontech). Examples of animal expression vectors include pEUK-Cl, pMAM, and pMAMneo (Clontech). A preferred recombinant expression vector includes the pMXT vector as shown in FIGS. 1A-1C.

The recombinant expression vector can be prepared using standard recombinant DNA techniques described in, for example, Sambrook et al., Molecular Cloning, a Laboratory Manual, 2d edition, Cold Spring Harbor Press, Cold Spring Harbor, N.Y. (1989), and Ausubel et al., Current Protocols in Molecular Biology, Greene Publishing Associates and John Wiley & Sons, New York, N.Y. (1994).

Desirably, the recombinant expression vector comprises regulatory sequences, such as transcription and translation initiation and termination codons, which are specific to the type of host (e.g., bacterium, fungus, plant, or animal) into which the vector is to be introduced, as appropriate and taking into consideration whether the vector is DNA- or RNA-based.

A construct of a recombinant expression vector, which is circular or linear, can be prepared to contain a replication system functional in a prokaryotic or eukaryotic host cell. Replication systems can be derived from ColEl, 2 μplasmid, λ, SV40, bovine papilloma virus, and the like. The recombinant expression vector of the invention can comprise a replication system, which comprises an oriP. Preferably, the inventive recombinant expression vector comprises an oriP, and not an Epstein Barr virus nuclear antigen (EBVNA), which EBVNA is known to activate an oriP.

As used herein, the term “oriP” or “Epstein-Barr virus origin of replication” refers to a nucleotide sequence that is substantially identical to the Epstein-Barr virus origin of replication, which has the nucleotide sequence of nucleotides 2561-4550 of SEQ ID NO: 1. It is preferred that no insertions, deletions, inversion, and/or substitutions are present in this nucleotide sequence. However, one of ordinary skill in the art appreciates that the nucleotide sequence of nucleotides 2561-4550 of SEQ ID NO: 1 can have insertions, deletions, inversion, and/or substitutions that will not negatively affect the function of the nucleotide sequence, which is to promote high copy episomal plasmid replication. One of ordinary skill in the art further appreciates that such high copy episomal plasmid replication occurs in mammalian cells.

The recombinant expression vector also preferably comprises a pUC19 origin of replication. As used herein, the term “pUC19 origin of replication” refers to the nucleotide sequence of the origin of replication from a pUC 19 vector, which is commercially available from Fermentas Life Sciences and has the nucleotide sequence of nucleotides 4551-5220 of SEQ ID NO: 1. It is preferred that no insertions, deletions, inversion, and/or substitutions are present in this nucleotide sequence. However, one of ordinary skill in the art appreciates that nucleotides 4551-5220 of SEQ ID NO: 1 can have insertions, deletions, inversion, and/or substitutions that will not affect the function of the nucleotide sequence, which is to promote high copy episomal plasmid replication. One of ordinary skill in the art further appreciates that such high copy episomal plasmid replication occurs in bacterial cells.

The recombinant expression vector can include one or more marker genes, which allow for selection of transformed or transfected hosts. Marker genes include biocide resistance, e.g., resistance to antibiotics, heavy metals, etc., complementation in an auxotrophic host to provide prototrophy, and the like. Suitable marker genes for the inventive recombinant expression vectors include, for instance, neomycin/G418 resistance genes, hygromycin resistance genes, histidinol resistance genes, tetracycline resistance genes, and ampicillin resistance genes.

The recombinant expression vector can comprise a native or normative promoter operably linked to the nucleic acid encoding the protein. The selection of promoters, e.g., strong, weak, inducible, tissue-specific, and developmental-specific, is within the ordinary skill in the art. Similarly, the combining of a nucleic acid with a promoter is also within the skill in the art. The promoter can be a viral promoter or a non-viral promoter. Preferably, the promoter is a viral promoter. More preferably, the viral promoter is a strong viral promoter, such as a cytomegalovirus (CMV) promoter. The CMV promoter is known in the art and has the nucleotide sequence of nucleotides 1-1037 of SEQ ID NO: 1. It is preferred that no insertions, deletions, inversion, and/or substitutions are present in this nucleotide sequence. However, one of ordinary skill in the art appreciates that nucleotides 1-1037 of SEQ ID NO: 1 can have insertions, deletions, inversion, and/or substitutions that will not affect the function of the nucleotide sequence, which is to drive the transcription of the recombinant protein coding sequence.

The recombinant expression vector comprises a 3′ UTR of a light chain gene. Preferably, the recombinant expression vector comprises a 3′ UTR of a light chain gene in combination with an Epstein-Barr virus origin of replication (oriP). As used herein, the term “3′ UTR” refers to a nucleotide sequence of a gene that is untranslated and is located 3′ to the stop codon of the coding sequence of that gene. The phrase “light chain gene” refers to a gene encoding a light chain of an immunoglobulin. Thus, in regard to the invention, the 3′ UTR of a light chain gene is a nucleotide sequence that is originally found in a light-chain gene and that is inserted into the inventive vector. The light chain gene can be a light chain gene of any mammal, such as a human, mouse, rat, goat, rabbit, horse, pig, etc. Preferably, the light chain gene is a mouse (murine) light chain gene. More preferably, the mouse light chain gene has the nucleotide sequence of nucleotides 1062-2560 of SEQ ID NO: 1. It is preferred that no insertions, deletions, inversion, and/or substitutions are present in this nucleotide sequence. However, one of ordinary skill in the art appreciates that nucleotides 1062-2560 of SEQ ID NO: 1 can have insertions, deletions, inversion, and/or substitutions that will not affect the function of the nucleotide sequence, which is to provide signals for polyadenylation. With respect to the inventive vectors, the 3′ UTR of a light chain gene is preferably located immediately 3′ to the stop codon of the coding sequence of the vector. If no coding sequence is present, then the 3′ UTR of a light chain gene preferably is located 3′ to the multiple cloning sequence and/or the CMV promoter. The recombinant expression vector can comprise a single copy of a 3′ UTR or multiple copies of a 3′ UTR. Preferably, the recombinant expression vector comprises a single copy of a 3′ UTR.

The recombinant expression vector preferably comprises a 5′ UTR intron. As used herein, the term “5′ UTR intron” refers to a nucleotide sequence that is transcribed but is removed by RNA splicing and thus not retained in the final transcript. It further is not translated and, thus, is not expressed as part of the protein, polypeptide, or peptide encoded by the vector. The 5′ UTR intron is preferably located after the promoter in the 5′ untranslated region of the recombinant expression vector. The 5′ UTR intron promotes enhanced expression. The 5′ UTR intron can be from any naturally-occurring source or can be constructed from portions of different sources, e.g., constructed from splice donor and acceptor sequences from different sources. For example, the 5′ UTR intron comprises a portion of a CMV intron and a portion of a SV40 16S intron. Preferably, the splice donor for the 5′ UTR intron is from the sequence downstream of the start of transcription from the viral promoter, and the splice acceptor is from the SV40 16S intron and has the nucleotide sequence of nucleotides 888-974 of SEQ ID NO: 1. It is preferred that no insertions, deletions, inversion, and/or substitutions are present in this nucleotide sequence. However, one of ordinary skill in the art appreciates that nucleotides 888-974 of SEQ ID NO: 1 can have insertions, deletions, inversion, and/or substitutions that will not affect the function of the nucleotide sequence, which is to drive the transcription of the recombinant protein coding sequence.

In a preferred embodiment, the recombinant expression vector comprises a 3′ UTR, an oriP, a pUC19 origin of replication, a viral promoter, and a 5′ UTR intron. Preferably, the viral promoter is a CMV promoter and the 5′ UTR intron comprises a portion of a CMV intron and a portion of a SV40 16S intron, e.g., comprises nucleotides 888-974 of SEQ ID NO: 1. Most preferably, the recombinant expression vector is the vector plasmid pMXT5, which is shown pictorially in FIG. 1A (pMXT5), and which has the nucleotide sequence (without any coding sequences) of SEQ ID NO: 1 (FIG. 10). For example, as shown in FIG. 10, the CMV promoter comprises nucleotides 1-1037; 5′ UTR intron comprises nucleotides 888-974; MCS comprises nucleotides 1038-1061; LC 3′ UT comprises nucleotides 1062-2560; OriP comprises nucleotides 2561-4550; pUC19 ori comprises nucleotides 4551-5220; and Ap comprises nucleotides 5221-6380.

The recombinant expression vector can be designed for either transient expression or for stable expression. Preferably, the vector of the invention promotes transient expression, i.e., is a recombinant transient expression vector, such that the vector is one that does not integrate into the genome of a host cell. Without being bound to any particular theory, it is believed that the recombinant expression vector can be made to be a transient expression vector by incorporating into the vector an oriP, which promotes high copy episomal plasmid replication.

The recombinant expression vector can comprise a nucleic acid sequence encoding any protein, such as a hormone, growth factor, antibody, receptor, structural protein, enzyme, etc. The protein can be, for example, a therapeutic protein, and can be naturally-occurring or non-naturally occurring, e.g., a genetically engineered protein including, for example, a fusion protein, a chimeric protein, etc. Preferably, the recombinant expression vector comprises such a nucleic acid for the expression of the protein. It is to be understood that the term “protein” as used herein includes parts or fragments thereof, and thus, polypeptides and peptides of any length are included within the meaning of this term. For example, polypeptides and peptides are included wherein the polypeptides can comprise, for instance, about 50 or more amino acids and the peptides can comprise, for instance, about 8-49 amino acids. The nucleic acid sequence encoding the protein can be obtained from any source, e.g., isolated from nature, synthetically generated, isolated from a genetically-engineered organism, and the like. An ordinarily skilled artisan will appreciate that any type of nucleic acid sequence (e.g., DNA, RNA, genomic DNA, and cDNA) that can be inserted into a recombinant expression vector can be used in connection with the invention. For example, the nucleic acid sequence encoding a protein can be naturally-occurring, e.g., a gene. Alternatively, the nucleic acid sequence encoding a protein can be non-naturally occurring, e.g., non-native to any organism, e.g., mammal. For instance, the nucleic acid sequence can be a codon optimized nucleic acid sequence in which codons within the nucleic acid sequence, which codons are not generally used by the host cell translation system, termed “rare codons,” are changed by in vitro mutagenesis to preferred codons without changing the amino acids of the synthesized protein (Bradel-Tretheway et al., J. Virol. Meth., 111: 145-156 (2003); Ramakrishna et al., J. Virol. 78: 9174-9189 (2004)). In addition, the nucleic acid sequence encoding a protein can be further modified, e.g., codon optimized, to improve the folding of the RNA, such that the folding of the RNA transcript encoded by the nucleic acid sequence is minimized. Whatever type of nucleic acid sequence is used, the nucleic acid sequence preferably encodes a secreted protein. By “secreted” is meant that the protein is released from the cell into the extracellular environment, thereby facilitating the purification of the protein. In this regard, the recombinant expression vector preferably comprises a signal sequence, which causes the expressed protein to be secreted from the cell by which it was expressed.

In a preferred embodiment, the recombinant expression vector comprises a nucleic acid encoding an immunogloblin chain, e.g., light chain or heavy chain. The immunoglobulin chain can be any immunoglobulin chain derived from any source, genetically-modified, or synthesized. Preferably, the immunoglobulin chain is a human immunoglobulin chain selected from the group consisting of a γ₁ heavy chain, a γ₂ heavy chain, a γ₄ heavy chain, a κ light chain, and a λ light chain. Exemplary heavy chain constant region sequences include: a γ₁ heavy chain constant region, which is encoded by the nucleotide sequence of SEQ ID NO: 4 and comprises the amino acid sequence of SEQ ID NO: 5; a γ₂ heavy chain constant region, which is encoded by the nucleotide sequence of SEQ ID NO: 6 and comprises the amino acid sequence of SEQ ID NO: 7; and a γ₄ heavy chain constant region, which is encoded by the nucleotide sequence of SEQ ID NO: 8 and comprises the amino acid sequence of SEQ ID NO: 9. Exemplary light chain constant region sequences include: a κ light chain constant region, which is encoded by the nucleotide sequence of SEQ ID NO: 10 and comprises the amino acid sequence of SEQ ID NO: 11, and a λ light chain constant region, which is encoded by the nucleotide sequence of SEQ ID NO: 12 and comprises the amino acid sequence of SEQ ID NO: 13. Exemplary antibody heavy and light chains include: an LDP-01 heavy chain, which is encoded by the nucleotide sequence of SEQ ID NO: 14 and comprises the amino acid sequence of SEQ ID NO: 15, and an LDP-01 light chain, which is encoded by the nucleotide sequence of SEQ ID NO: 16 and comprises the amino acid sequence of SEQ ID NO: 17. The LDP-01 antibody is referred to herein as Ab#1 and has been described in WO 2004/033693 (PCT/US2003/010154) and U.S. Patent Application Publication No. 2003/0203447 A1.

In this regard, the recombinant expression vector desirably comprises an antibody signal sequence, which promotes the secretion of the antibody into the extracellular environment. Suitable antibody signal sequences are known in the art. For example, a preferred signal sequence comprises SEQ ID NO: 2 or SEQ ID NO: 3.

The recombinant expression vector can alternatively comprise a nucleic acid sequence encoding a functional fragment of a protein. The term “functional fragment” which is synonymous with “functional part” or “functional portion,” when used in reference to a protein, refers to any part or fragment of the protein, which part or fragment retains a biological activity of the protein of which it is a part. Functional fragments encompass, for example, those parts of a protein (the parent protein) that retain a function of the parent protein to a similar extent, the same extent, or to a higher extent, as the parent protein. For instance, if the protein is an immunoglobulin, functional fragments thereof can include any portion of the immunoglobulin which, for example, retains the ability to bind to the antigen of the parent immunoglobulin. Also, for example, if the protein is a cell surface receptor, functional fragments thereof can include any portion of the cell surface receptor which, for instance, retains the ability to bind to the ligand of the parent cell surface receptor. In reference to the parent protein, the functional fragment can comprise, for instance, about 10%, 25%, 30%, 50%, 60%, 80%, 90%, 95%, or more of the parent protein. The functional portion can comprise additional amino acids at the amino or carboxy terminus of the portion, or at both termini, which additional amino acids are not found in the amino acid sequence of the parent protein. Desirably, the additional amino acids do not interfere with the biological function of the functional portion.

The invention further provides a host cell comprising any of the recombinant expression vectors described herein. As used herein, the term “host cell” refers to any type of cell that can contain the inventive recombinant expression vector. The host cell can be a eukaryotic cell, e.g., plant, animal, fungi, or algae, or can be a prokaryotic cell, e.g., bacteria or protozoa. The cell can be a cultured cell or a primary call, i.e., isolated directly from an organism, e.g., a human. The cell can be an adherent cell or a suspended cell, i.e., a cell that grows in suspension. Suitable host cells are known in the art and include, for instance, DH5a E. coli cells, Chinese hamster ovarian (CHO) cells, monkey VERO cells, COS cells, HEK293 cells, and the like. For purposes of amplifying or replicating the recombinant expression vector, the host cell is preferably a prokaryotic cell. More preferably, the host cell is a DH5α cell. For purposes of producing a recombinant protein, the host cell is preferably a mammalian cell. Most preferably, the host cell is a human cell. While the cell can be any cell of the human body, it is preferred that the cell is a human embryonic kidney cell. More preferred is that the human embryonic kidney cell expresses an Epstein Barr virus nuclear antigen-1 (EBNA-1) protein, e.g., a 293E cell.

As used herein, the term “mammal” refers to any mammal, including, but not limited to, mammals of the order Rodentia, such as mice and hamsters, and mammals of the order Logomorpha, such as rabbits. It is preferred that the mammals are from the order Camivora, including Felines (cats) and Canines (dogs). It is more preferred that the mammals are from the order Artiodactyla, including Bovines (cows) and Swines (pigs) or of the order Perssodactyla, including Equines (horses). It is most preferred that the mammals are of the order Primates, Ceboids, or Simoids (monkeys) or of the order Anthropoids (humans and apes). An especially preferred mammal is the human.

The invention further provides methods of producing a recombinant protein. In a first method, the recombinant protein is a heterodimeric or heteromultimeric protein comprising a first polypeptide chain and a second polypeptide chain, wherein the first polypeptide chain is different from the second polypeptide chain. The first method comprises contacting cells in a medium with a first vector and a second vector, wherein the first vector encodes the first polypeptide chain and the second vector encodes the second polypeptide chain, and the second vector is present in the medium in an amount which is about 1.5 to about 2.5 times as much as the amount of the first vector, whereupon a recombinant protein is produced. The first and second vectors can be any suitable vector and preferably are inventive recombinant expression vectors as described herein.

For purposes of the first inventive method of producing a protein, the first and second vectors can independently be any type of vector, i.e., the first and second vectors can have the same regulatory elements but differ only in the recombinant protein coding sequence contained therein. By way of example, both the first vector and second vector can be the pMXT vector as shown in FIG. 1A. Preferably, each of the first vector and the second vector is one of the inventive recombinant expression vectors described herein. Most preferably, the first and second vectors are pMXT vectors. For example, it is preferred that each of the first and the second vector is a recombinant transient expression vector. It is also preferred that each of the first and second vector comprises a 3′ UTR of a light chain gene and an oriP. It is also preferred that each of the first and second vector comprises a viral promoter, a pUC19 origin of replication, a 5′ UTR intron, or a combination of any of the foregoing. Preferably, the viral promoter is a CMV promoter, and the 5′ UTR intron comprises nucleotides 888-974 of SEQ ID NO: 1. Moreover, it is preferred that each of the first and second vector comprises an antibody signal sequence.

Also, with respect to the first inventive method of producing a protein, the second vector is present in the medium in an amount which is about 1.5 to about 2.5, e.g., 1.6, 1.7, 1.75, 1.8, 1.9, 2.0, 2.125, 2.25, 2.3, 2.4, and 2.5, times as much as the amount of the first vector. Preferably, the second vector is present in the medium in an amount which is about 1.75 to about 2.25 times as much as the amount of the first vector. More preferably, the second vector is present in the medium in an amount which is about twice as much as the amount of the first vector.

The invention further provides a second method of producing a recombinant protein. The second method comprises culturing cells, which have been contacted with a recombinant transient expression vector encoding the protein, in a medium in a membrane-enhanced culturing vessel, whereupon a recombinant protein is produced. The second method can alternatively comprise culturing cells, which have been contacted with a recombinant transient expression vector encoding the recombinant protein, in a medium in a Fembach flask, whereupon a recombinant protein is produced. The recombinant transient expression vector can be any suitable such vector and preferably is an inventive recombinant expression vector as described herein.

In a third method, the recombinant protein is produced upon contacting cells with at least one of the inventive recombinant expression vectors described herein. In a fourth method, the recombinant protein is produced upon culturing any of the inventive host cells comprising any of the inventive recombinant expression vectors described herein.

Any suitable method can be employed to contact cells with a first vector, a second vector, or a recombinant expression vector, such that the cells express the protein encoded by the vector. Methods of contacting cells, such that the cells are modified to express a particular protein, polypeptide, or peptide, are well-known in the art. See the references listed in Sambrook et al. (1989), supra. Suitable methods of contacting cells to this end include, for instance, infection with a viral vector, transfection with a lipofection reagent, cationic polymer, DEAE, or calcium phosphate, and electroporation.

The cells can be contacted with a first vector, a second vector, or a recombinant expression vector in the presence of a suitable cationic polymer. Suitable cationic polymers for transfecting cells are known in the art, and include, for example, polylysine and polyethyleneimine (PEI). In a preferred embodiment of the inventive method, the cationic polymer is PEI. PEI can be linear or branched and can vary in molecular weight, depending on the number of base units, which comprise the polymer. Preferably, the PEI is a linear PEI. More preferably, the linear PEI has a molecular weight of about 25 kDa. Although the amount of PEI used in the method can be any amount, it is preferred that the linear PEI is present in an amount that is about 1.5 to about 4.5, e.g., 1.5, 1.6, 1.75, 2.0, 2.25, 2.5, 2.6, 2.7, 2.75, 2.8, 2.9, 3.0, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.75, 3.8, 3.9, 4.0, 4.1, 4.25, 4.3, 4.4, and 4.5, times the amount of the vector(s) contacting the cells. Preferably, the PEI is present in an amount that is about 2.5 to 3.5 times the amount of the vector(s) contacting the cells. More preferably, the PEI is present in an amount that is about twice the amount of the vector(s) contacting the cells.

For purposes of the inventive method comprising contacting cells with more than one vector, e.g., a first vector and a second vector, the cells can be contacted with the first vector and second vector in a sequential fashion, e.g., first vector contacted with the cells before the second vector. Alternatively, the cells can be contacted with the first vector and second vector simultaneously. Preferably, the cells are contacted with the first vector and second vector simultaneously. For example, in a method comprising contacting cells with more than one vector, the cells can be contacted with a first vector before or simultaneously with a second or additional vector.

As used herein, the term “culturing” is synonymous with “maintaining.” Methods of culturing cells are known in the art (see, e.g., Tissue Engineering Methods and Protocols, Morgan and Yarmush (eds.), Humana Press, Inc., Totowa, N.J., 1999). As one ordinarily skilled recognizes, the conditions under which cells are cultured varies depending on the cell type. The conditions include temperature of the environment, the culturing vessel containing the cells, the composition of the various gases, e.g., CO₂, which comprises the cell culture atmosphere or environment, the medium in which the cells are maintained, the components and pH of the medium, the density at which cells are maintained, the schedule by which the medium needs to be replaced with new medium, etc. These parameters are often known in the art or can be empirically determined. For example, with respect to the inventive methods, wherein cells are cultured in a medium, e.g., a first medium, a second medium, etc., any method can be employed to culture the cells in the medium, such that the cells express (and, in some instances, secrete) the protein encoded by the vector, which was contacted to the cells.

The cells are desirably cultured in a membrane-enhanced culturing vessel or a Fembach flask. For purposes herein, the term “membrane-enhanced culturing vessel” refers to a container for holding cell cultures that have been improved upon by the addition of at least one membrane. Suitable membrane-enhanced culturing vessels include membrane-based cell culture vessels, dialysis-based cell culture vessels, membrane-based high density cell culture vessels, and two-compartment vessels. The term “vessel” as used herein is synonymous with systems, reactors, bioreactors, flasks, and devices. Suitable membrane-enhanced culturing vessels include, for instance, miniPerm® flasks, OptiCell® flasks, and the CELLINE™ CL1000 (referred to herein as Integra flasks or Integra CL1000 flasks), which are commercially available from companies, such as IBS Integra Biosciences AG (Chur, Switzerland), OptiCell (Westerville, Ohio), VWR, Fisher Scientific, and Labmate (Asia). Most preferably, the membrane-enhanced culturing vessel is an Integra CL1000. For example, one of the ordinary skill in the art appreciates that a membrane-enhanced culturing vessel such as an Integra flask may comprise a nutrient chamber and a cultivation chamber, wherein nutrients from a media reservoir in the nutrient chamber pass through a semi-permeable membrane into the cultivation chamber containing cells so as to provide a continuous supply of nutrients and wherein the membrane also allows for diffusion of metabolites out of the cultivation chamber and away from contact with the cells but does not permit diffusion of a recombinant protein produced by the cells (e.g., an antibody or antibody fragment) out of the cultivation chamber, and further wherein the cells also have sufficient gas exchange such as access to oxygen and carbon dioxide through a separate silicone membrane at the bottom of the vessel.

As used herein, the term “Fembach flask” refers to a commercially available Corning® polycarbonate Erlenmeyer flask having the Fembach design. Such flasks are commercially-available from companies such as Life Sciences.

Without being bound to any particular theory, membrane-enhanced flasks (e.g., Integra CL1000, OptiCell® flasks, and miniPerm® flasks) and Fernbach flasks are particularly suitable for culturing transfected cells, for example, transiently transfected cells, as these devices permit efficient gas exchange between the cells and the environment, e.g., the incubator environment, which permits optimal cell growth and production of the recombinant protein. Under certain conditions, shake flasks can also be suitable culturing vessels in which cells can be cultured for optimal cell growth and production of the recombinant protein. It should be understood that any flask or culturing vessel that permits efficient gas exchange between the cells and the environment are included in the scope of the invention and are not limited to only the aforementioned flasks and culturing vessels.

In the inventive methods comprising culturing cells, the medium can be any suitable medium for culturing cells known in the art. The medium can be, for example, a culture medium containing 1% low immunoglobulin (Ig) fetal bovine serum (FBS). Alternatively, the medium can be a serum-free cell culture medium, e.g., IS293™ medium. In some instances, the medium is preferably a serum-free IS293™ medium (Irvine Scientific, Irvine, Calif.).

The cell cultures of the inventive methods can be initiated or seeded at any suitable cell density. As one of ordinary skill in the art recognizes, the seeding density depends on a variety of factors, such as cell type, culturing conditions, and the day which has been selected for harvesting or purifying the recombinant protein from the cell culture. Desirably, the cell density is within the range of about 1.0×10⁶ to about 2.0×10⁷ (e.g., about 1.0×10⁶ to about 1.5×10⁷). More preferably, the initiating seeding cell density of the cell culture is about 3.0×10⁶ to about 1.0×10⁷. Without being bound to any particular theory, it is believed that the seeding density of cells, which have been transiently transfected with a vector encoding a protein, is a factor in obtaining efficient production of a recombinant protein.

For purposes of the inventive methods, the cells that are cultured or are contacted with a first vector, a second vector, or a recombinant expression vector can be any cell, such as those described herein as “host cells.” For example, the cells that are cultured and/or contacted with one or more than one recombinant expression vector can be any host cells. Preferably, the cells are mammalian cells, and, more preferably, the cells are human cells. The cells are desirably human embryonic kidney cells. In a most preferred embodiment, the human embryonic kidney cells express Epstein-Barr virus nuclear antigen-1 protein (EBNA-1), e.g., 293E cells.

Cells, which have been contacted with a recombinant transient expression vector, can be obtained by transiently transfecting cells by any method known in the art, including those described herein. Recombinant transient expression vectors are known in the art and include, for instance, pCEP4, pcDNA3, and any of the recombinant expression vectors described herein which comprise an oriP. Preferably, the recombinant transient expression vectors are pMXT vectors. For example, the vectors can be any of the inventive recombinant expression vectors as described herein.

With respect to the first method of producing a recombinant protein (e.g., comprising contacting cells with a first vector and a second vector), the method can further comprise the second inventive method of producing a recombinant protein. That is, the method of producing a recombinant protein can further comprise the step of culturing the cells, which have been contacted with a first vector and a second vector, in a second medium in a membrane-enhanced culturing vessel (e.g., an Integra CL1000, an OptiCell® flask, a miniPerm® flask), a Fernbach flask, or like flask. In such an embodiment, the second medium can be different from the medium in which the first and second vectors are present. For purposes of the methods, which comprise culturing cells in a membrane-enhanced culturing vessel, a Fembach flask, or like flask, the suitable medium for use in such a vessel or flask can be a serum-free cell culture medium, e.g., IS293 medium. Preferably, the medium is serum-free IS293 medium (Irvine Scientific, Irvine, Calif.).

With respect to the second inventive method of producing a recombinant protein, the method can comprise the first inventive method of producing a recombinant protein. One of ordinary skill in the art recognizes that the methods described herein can be combined in such a way, such that all of the limitations of the methods are met. Such a combined method is within the scope of the invention.

With respect to any of the inventive methods comprising culturing cells, e.g., in a membrane-enhanced culturing vessel, a Fernbach flask, or like flask, the method can further comprise purifying or isolating the recombinant protein from the medium, e.g., the serum-free medium. As used herein, the terms “purifying” and “isolating” do not necessarily refer to absolute purity or isolation, as one of ordinary skill in the art appreciates that a partially purified or partially isolated protein can be useful or of value.

Methods of purifying proteins from mixtures are known in the art. Suitable purification methods include, for example, chromatography, electrophoresis, and the like. Suitable chromatographic methods of purifying polypeptides include, for example, HPLC, ion-exchange chromatography, affinity chromatography, etc. Preferably, the purifying comprises chromatographing the medium through a resin, such as a cationic resin, an anionic resin, and an affinity resin. If the polypeptide is an immunoglobulin chain, the purifying preferably comprises the use of resin comprising Staphylococcus aureus Protein A, which is a bacterially-produced protein that binds to the Fc regions of IgG antibodies. More preferably, the purifying comprises centrifuging the medium through a column comprising Protein A, e.g., centrifuging the medium through a Protein A spin column (which is commercially available from Pro-Chem).

The purifying can occur at any point in time after culturing the cells, which have been contacted with a vector. In some instances, it is preferable for the purifying to occur after about 3 days of culturing, e.g., after about 3, 4, 5, 6 or more days. In other instances, it is preferable for the purifying to occur after about 7 days of culturing, e.g., after about 7, 8, 9, 10, 11, 12, 13, 14, 15 or more days.

The invention provides fast and efficient methods of producing high levels of recombinant proteins. In some instances, at least 300 μg/ml recombinant protein is produced after 3 days of culturing. In other instances, at least 500 μg/ml recombinant protein is produced after 3 days of culturing. In some preferred instances, at least 700 μg/ml recombinant protein is produced after 3 days of culturing.

The term “recombinant protein” as used herein, refers to any protein or part thereof that is produced by a genetically-engineered organism. For example, the recombinant protein can be any of the proteins described herein.

For purposes of the first method of producing a recombinant protein, the recombinant protein is a heterodimeric protein or a heteromultermeric protein, such as a tetramer, which comprises two copies of two different polypeptide chains. Such proteins are known in the art, and include, for instance, hemoglobin, immunoglobulins, T cell receptors, and B cell receptors, etc. In a preferred embodiment of the first inventive method, the recombinant protein is a heterotetrameric protein. Desirably, the heterotetrameric protein is an immunoglobulin. In this instance, it is preferred that the first vector encodes a heavy chain of an immunoglobulin, or a part thereof, and the second vector encodes a light chain of an immunoglobulin, or a part thereof. The heavy chain can be any heavy chain of any immunoglobulin, as described herein. The light chain can be any light chain of any immunoglobulin, as described herein. Exemplary antibody heavy and light chains: an LDP-01 heavy chain, which is encoded by the nucleotide sequence of SEQ ID NO: 14 and comprises the amino acid sequence of SEQ ID NO: 15, and an LDP-01 light chain, which is encoded by the nucleotide sequence of SEQ ID NO: 16 and comprises the amino acid sequence of SEQ ID NO: 17. The LDP-01 antibody is referred to herein as Ab#1 and has been described in WO 2004/033693 (PCT/US2003/010154) and U.S. Patent Application Publication No. 2003/0203447 A1.

EXAMPLES

The following examples further illustrate the invention but, of course, should not be construed as in any way limiting its scope.

Example 1

This example demonstrates the construction of recombinant expression vectors of the invention.

Transient expression vectors for expression of any gene were constructed with a multilinker site containing unique restriction sites positioned between the 3′ end of the CMV promoter and the 5′ end of the mouse light chain 3′ untranslated region. Transient expression vectors containing cDNAs, which encode light chain κ or λ genes or heavy chain γ₁, γ₂, or γ₄ genes, under the control of a CMV promoter (Boshart et al., Cell 41: 521-530 (1985)) and mouse light chain 3′ untranslated region (Xu et al., J. Biol. Chem. 261: 3838-3845 (1986)) were contructed. Unique restriction sites were positioned at the 5′ end of the V region (e.g., SalI) and in the junction regions between the V and constant regions (BlpI for heavy chain, BsiWI for κ light chain and AvrII for lambda) for the cloning of any new V region adjacent to the desired cognate constant region. The vectors also contained the Epstein Barr virus oriP sequence (Reisman et al., Mol. Cell. Biol. 5: 1822-1832 (1985)) for episomal plasmid replication in 293E cells, the origin of replication from the vector pUC19, and the gene encoding resistance to ampicillin for selection of tranformants in E. coli. The transient expression vectors containing the multilinker sites, the heavy chain, and the light chain are shown in FIGS. 1A-1C.

Example 2

This example demonstrates a method of transiently transfecting cells for producing recombinant proteins.

293E cells (Invitrogen, R620-07) were maintained as adherent cultures in Dulbecco's Modified Eagle Medium (DMEM) (Gibco-Invitrogen) supplemented with 10% fetal bovine serum (FBS, Hyclone), 2 mM glutamine, and 250 μg/ml G418 antibiotic (Gibco-Invitrogen). For growth in suspension culture, the cells were adapted to the following serum-free media formulations: IS293™ (Irvine Scientific), IS293-V™ (Irvine Scientific), 293 SFM II (Gibco-Invitrogen), H-SFM (Gibco-Invitrogen), and HYQ®PF293 (HyClone). The cells were originally supplemented with 10% low IgG FBS (HyClone) and 2 mM glutamine and gradually weaned down to 1% low IgG FBS over a period of several weeks. Once in 1% low IgG FBS, the cells were transferred to shake flasks for continued adaptation to suspension growth. Growth and viability were monitored using the VICELL™ XR Cell Viability Analyzer (Beckman-Coulter).

All plasmids were transformed into DH5a cells (Invitrogen) and purified using endotoxin-free plasmid purification kits (QIAGEN®). For transfections in 6-well plates, 2 ml of cells at 5×10⁵ cells/ml were seeded per well. For transfections in shake flask cultures, cells were seeded at 8×10⁵ cells/ml at the appropriate volumes prior to transfection. DNA (2 μg/ml) was pre-incubated with linear polyethyleneimine (PEI, 25 kDa MW, Polysciences) at a concentration of 4 μg/ml for 10 min at room temperature prior to addition to cells. The DNA/PEI mixture was then added to cells, and the cells with the DNA/PEI were either maintained in shake flasks or transferred to Integra flasks.

Example 3

This example demonstrates the determination of the optimal PEI:DNA ratio for transient transfections.

Adherent 293E cells grown in DMEM supplemented with 10% FBS in 6-well plates were transfected with pQBI-pGK (GFP expressing plasmid, Q-biogene) using linear polyethyleneimine (PEI) as described in Example 2. DNA (1 μg/ml, 2 μg/ml, or 5 μg/ml) was pre-incubated with linear PEI (1, 2, 4, 5, 10, or 25 μg/ml) for 10 min at room temperature prior to the addition to cells, then the PEI/DNA mixture was added to cells, and the cells were maintained in shake flasks or Integra flasks.

GFP expression was monitored 24 hours post-transfection using a Becton Dickinson FACScan flow cytometer equipped with the Cytek Automated Microsampler System (AMS) 96-well plate reader. Flow data was analyzed using FlowJo (Tree Star, Inc.). Cells also were counterstained with 1 μg/ml propidium iodide (PI) to determine cell viability. Growth and viability of the cells post-transfection were monitored using the VICELL™ XR Cell Viability Analyzer (Beckman-Coulter). The cells transfected with 1 μg/ml DNA and 1 μg/ml PEI (FIG. 2A); 2 μg/ml DNA and 2 μg/ml PEI (FIG. 2B); 5 μg/ml DNA and 5 μg/ml PEI (FIG. 2C); 1 μg/ml DNA and 2 μg/ml PEI (FIG. 2D); 2 μg/ml DNA and 4 μg/ml PEI (FIG. 2E); 5 μg/ml DNA and 10 μg/ml PEI (FIG. 2F); 1 μg/ml DNA and 5 μg/ml PEI (FIG. 2G); 2 μg/ml DNA and 10 μg/ml PEI (FIG. 2H); and 5 μg/ml DNA and 25 μg/ml PEI (FIG. 2I) were measured for GFP expression α-axis) and PI staining (y-axis) by flow cytometry, and the resulting data was plotted in the series of graphs of FIGS. 2A-2I.

As shown in FIGS. 2A-2I, the DNA concentration of 1 μg/ml at a PEI:DNA ratio of 2:1 gave the highest percentage of cells expressing GFP with relatively low cellular cytotoxicity 24 hours post-transfection.

The results of this example demonstrated the production of recombinant protein and confirmed that the optimal PEI:DNA ratio for transient transfection is 2:1.

Example 4

This example demonstrates the determination of the optimal medium for culturing transiently transfected cells.

293E cells were grown and transfected in the presence of 1% low-IgG serum in 6-well plates and shake flasks, as described in Example 2. Twenty-four hours after transfection, 293E cells were adapted to suspension growth in one of 5 different serum-free media formulations (IS293™, H-SFM, IS293V™, SFMII, or HYQ®PF293) or one serum-containing media formulation (DMEM) as in Example 2.

Twenty-four to forty-eight hours later, GFP expression by transfected cells were determined as described in Example 3. Cells also were counterstained with 1 μg/ml PI to determine cell viability. Growth and viability of the cells post-transfection were monitored using the VICELL™ XR Cell Viability Analyzer (Beckman-Coulter). The resulting data from GFP expression (bars) and for PI staining (X) was plotted to form the graph of FIG. 3.

As shown in FIG. 3, IS293™ medium (Irvine Scientific) gave the highest percentage of GFP-expressing cells with minimal cytotoxicity in shake flasks; values were comparable to those obtained with adherent 293E cells cultured in DMEM supplemented with 10% FBS.

The results of this example demonstrated that IS293™ medium is the optimal serum-free medium to be used with transiently transfected cells for producing recombinant proteins.

Example 5

This example demonstrates the determination of optimum heavy and light chain plasmid ratios for maximum antibody productivity.

Various ratios of pMXT (heavy chain (HC)):pMXT (light chain (LC)) (see Example 1) or pCEP4 (HC):pCEP4 (LC) were tested for effects on antibody productivity in cells grown in IS293™ medium supplemented with 1% low-IgG serum in shake flasks. The pCEP4 vector containing the nucleotide sequence encoding the Ab# 1 heavy chain (SEQ ID NO: 14) was constructed by cloning the coding sequence into KpnI and Xho sites. The pCEP4 vector containing the nucleotide sequence encoding the Ab#1 light chain (SEQ ID NO: 16) was constructed by cloning the coding sequences into Nhe and Xho sites. The encoded heavy chain and light chain of Ab#1 is set forth as SEQ ID NOs: 15 and 17, respectively. All plasmids were amplified by transformation into DH5 α cells and purified as described in Example 2. 293E cells were transiently transfected as described in Example 2. Transfected cells were transferred to IS293™ medium in shake flasks for 7-10 days. Antibody expression by the cells transfected with a 1:1, 1:2, or 2:1 ratio of vector encoding HC:vector encoding LC, wherein the vectors were either pMXT or pCEP4 was determined by sandwich ELISA and the data analyzed in PRISM™ (GraphPad). The resulting data was plotted to form the graph of FIG. 4.

As shown in FIG. 4, a 1:2 ratio of HC:LC generated the highest antibody productivity with Ab#1 achieving levels of 60-70 μg/ml after 7-10 days. The highest productivity for Ab#1 (LDP-01) in pMXT was ˜3× greater than the best output achieved using pCEP4.

The results of this example demonstrated that the pMXT vector is optimal for co-transfecting cells with vectors encoding different polypeptide chains at a ratio of 1:2.

Example 6

This example demonstrates that the level of antibody production by transiently transfected cells cultured post-transfection in membrane-enhanced culturing vessels are comparable to the level of antibody production achieved by transfected cells cultured post-transfection in shake flasks.

293E cells were transiently transfected in shake flasks as described in Example 2. Cells were either maintained in the shake flasks or transferred to 15 ml of medium and placed in Integra CL1000 flasks. After 7-10 days, cell culture supernatant was harvested, clarified, and purified for antibodies using a standard Protein A column, if cells were cultured in shake flasks, or a Protein A spin column, if cells were cultured in Integra flasks. Cell viability and antibody production of both sets of cells 0, 4, 7, and 14 days post-transfection were assayed as described in Examples 4 and 5, respectively. For antibody expression using the Integra CL1000 flask, 200 ml of transfected 293E cells were resuspended in 15 ml of IS293™ medium supplemented with 1% low IgG FBS and 250 μg/ml G418 antibiotic and transferred into the membrane compartment. One liter of IS293™ medium was added to the upper media chamber.

The cell viability (X) and antibody production (●) of the transfected cells maintained in Integra flasks are shown in FIG. 5A, whereas the levels of antibody production for Ab#1 by cells maintained in either shake flasks or by Integra CL1000 flasks are shown in FIG. 5B.

As shown in FIG. 5A, antibody production of cells cultured in Integra flasks peaked at 7 days, producing over 1 mg/ml antibody. This level is comparable to the level of antibody production of transiently transfected cells cultured in shake flasks as shown in FIG. 5B.

The results of this example demonstrated that Integra flasks are suitable culturing vessels for maintaining small volumes of transiently transfected cells. The small volume permits the use of Protein A spin columns, which facilitates the purification of antibodies from the cell culture supernatant.

Example 7

This example demonstrates a method of producing antibodies in membrane-enhanced culturing vessels at optimized seeding densities.

Suspension-adapted HEK 293E cells were maintained in IS293™ medium (Irvine Scientific) supplemented with 1% low IgG FBS (HyClone), 2 mM glutamine (Gibco-Invitrogen), and 250 μg/ml G418 antibiotic (Gibco-Invitrogen). For transfection, cells were seeded at 8×10⁵ cells/ml in shake flasks at the appropriate volumes prior to transfection. DNA encoding Ab#1 or Ab#2 (which differed from Ab#1) was pre-incubated with linear polyethyleneimine (PEI, 25 kDa MW, Polysciences) at optimized conditions (see, e.g., Example 3; see also, e.g., Handa et al., American Society for Cell Biology, poster presentation #1937 (2004)) prior to addition to cells. For antibody expression using the Integra CL1000 flask, cells at the following seeding densities were resuspended in 30 ml of IS293™ medium supplemented with 1% low IgG FBS, 2 mM glutamine, and 250 μg/ml G418 antibiotic and transferred into the cultivation chamber: 1.3×10⁶ (I-50), 2.7×10⁶ (I-100), 5.3×10⁶ (I-200), and 1.1×10⁷ (I-400). For comparison, 8×10⁵ cells (E-200) were seeded in Erlenmeyer flasks. All flasks were incubated for 3, 5, 7, or 10 days post-transfection. One ml samples from the nutrient chambers and cultivation chambers of the Integra CL 1000 flasks were removed and analyzed at 3, 5, 7, or 10 days post-transfection.

Growth and viability were monitored using the VICELL™ XR Cell Viability Analyzer (Beckman-Coulter). The percentage of viable cells 1, 3, 5, 7, and 10 days after transfection for cells tranfected at different seeding densities is shown in FIGS. 6A and 6B. The viable cell count of cells 0, 1, 3, 5, 7, and 10 days after transfection for cells transfected at different seeding densities is shown in FIGS. 7A and 7B.

As shown in FIGS. 6A (Ab#1), 6B (Ab#2), 7A (Ab#1) and 7B (Ab#2), cell viability did not vary between flasks, but viable cell growth was improved in the Integra flasks for all seeding densities tested. Maximum densities of 3-5×10⁷ cells/ml were achieved for all conditions over the 10 day analysis period.

Analytes, gases, and pH of the samples were determined 3, 5, 7, and 10 days post-transfection using a BIOPROFILE™ Chemistry Analyzer (Nova Biomedical). The data for selected nutrients and metabolites of the media containing cells producing Ab#1 or the media containing no cells (Media Only) are set forth in Table 1. TABLE 1 Flask Media Day Day Day Day Nutrient Sample Chamber Only 3 5 7 10 Glucose I-50 cultivation 5.37 3.35 2.85 2.57 2.76 (g/L) I-50 nutrient 5.36 5.11 4.49 4.12 3.47 I-100 cultivation 5.37 3.86 2.93 2.99 2.41 I-100 nutrient 5.36 5.05 4.39 3.97 3.85 I-200 cultivation 5.37 3.27 3.09 3.01 2.84 I-200 nutrient 5.36 4.62 4.12 3.68 3.10 I-400 cultivation 5.37 2.86 3.11 2.98 2.74 I-400 nutrient 5.36 4.52 3.94 3.67 3.23 E-200 N/A 5.36 4.42 2.51 2.36 2.06 Glutamine I-50 cultivation 6.96 6.07 4.77 4.80 4.32 (mmol/L) I-50 nutrient 6.82 6.30 5.89 5.54 4.79 I-100 cultivation 6.96 5.84 4.57 4.65 4.03 I-100 nutrient 6.82 6.37 5.88 5.41 5.10 I-200 cultivation 6.96 5.36 4.64 4.50 4.68 I-200 nutrient 6.82 6.07 5.62 5.19 4.63 I-400 cultivation 6.96 5.47 5.25 5.02 4.21 I-400 nutrient 6.82 6.37 5.91 5.68 5.12 E-200 N/A 6.82 6.31 5.82 5.53 5.08 Lactate I-50 cultivation 0.26 1.92 2.42 3.17 2.82 (g/L) I-50 nutrient 0.34 0.74 1.36 1.82 2.27 I-100 cultivation 0.26 1.85 2.43 2.74 2.82 I-100 nutrient 0.34 0.99 1.62 1.94 2.52 I-200 cultivation 0.26 2.47 2.32 2.84 2.72 I-200 nutrient 0.34 1.25 1.86 2.09 2.33 I-400 cultivation 0.26 2.57 2.31 2.78 2.66 I-400 nutrient 0.34 1.63 2.17 2.35 2.38 E-200 N/A 0.34 1.53 2.50 2.59 2.48

As shown by the resulting data, transiently transfected cells maintained in 30 ml media in the cultivation chamber of an Integra CL1000 flask can reach cell densities of up to 3-5×10⁷ viable cells/ml (e.g., 4.5×10⁷). Nutrients from the media reservoir in the nutrient chamber pass through a semi-permeable membrane into the cultivation chamber providing a continuous supply of essential nutrients. The membrane also allows for diffusion of metabolites out of the cultivation chamber and away from contact with cells. Cells also have efficient access to oxygen and carbon dioxide through a separate silicone membrane at the bottom of the flask.

The Integra supernatant from the cultivation chamber had higher glucose levels than shake flasks but lower glutamine levels. The levels of lactate appeared similar between the two cultures. The higher relative levels of glucose to lactate in the Integra cultures could indicate that the cells are generating more ATP by promoting efficient entry of pyruvate from glycolysis into the TCA cycle.

Antibody titers of transfected cells placed in Integra flasks or shake flasks at different seeding densities were determined using the EASY-TITER™ Human IgG Assay Kit (Pierce) 0, 3, 5, 7, and 10 days post-tranfection. The data expressed as the concentration of antibody titers (μg/ml) is shown in FIGS. 8A (Ab#1) and 8B (Ab#2), whereas the data expressed as the total antibody yield (mg) is shown in Table 2. TABLE 2 Day 3 Day 5 Day 7 Day 10 % of E-200 % of E-200 % of E-200 % of E-200 Sample Yield (mg) Max Yield Yield (mg) Max Yield Yield (mg) Max Yield Yield (mg) Max Yield Ab#1 E-200 7.6 52% 14.5 100% 7.4 51% 7.6  52% I-50 0.8  6% 5.2  36% 8.5 59% 13.3  92% I-100 5.2 36% 8.3  57% 14.7 101%  24.0 166% I-200 9.4 65% 13.8  95% 13.8 95% 19.0 131% I-400 11.00 76% 15.7 108% 14.3 99% 11.7  81% Ab#2 E-200 4.1 50% 6.3  77% 7.5 92% 8.2 100% I-50 1.2 15% 4.3  52% 7.6 93% 6.4  78% I-100 3.5 43% 8.3 101% 10.4 127%  12.9 157% I-200 6.1 74% 13.0 159% 17.0 207%  22.0 268% I-400 10.8 132%  17.8 217% 21.0 256%  22.0 268%

As shown in FIGS. 8A and 8B, antibody productivity for the two antibodies tested, Ab#1 and Ab#2, were different in Erlenmeyer flasks. Ab#1 peaked early at day 5 (˜70 μg/ml), followed by a decrease in antibody concentration. Ab#2 showed slower and steadier productivity over the full 10 days, achieving a maximal antibody output of ˜40 μg/ml. For both Ab#1 and Ab#2, antibody productivity in the Integra flasks achieved steady levels of increasing Ab productivity over the 10 day period. An exception was the 1-400 sample for Ab# 1, which showed slight decreases in productivity at days 7 and 10; however, the decrease was substantially less as compared to that in the E-200 samples.

As shown in Table 2, the maximal yield for Ab#1 in the E-200 culture was ˜15 mg at day 5. Comparable levels (≧90% of E-200 maximum) were obtained in the I-200 and I-400 cultures at day 5 as well, and higher total yields were obtained with I-100 and I-200 after day 10, namely 166% (24 mg) and 131% (19 mg), respectively.

As also shown in Table 2, the maximal yield for Ab#2 in the E-200 culture was ˜8 mg at day 10. Comparable levels (≧90% of E-200 maximum) were obtained as early as day 3 with I-400 (˜11 mg), at day 5 with I-100 and I-200 (˜8 mg and ˜13 mg, respectively), and day 7 with I-50 (˜8 mg). Higher yields were obtained for I-100, I-200, and I-400 at day 10, 157% (13 mg), 268% (22 mg), and 268% (22 mg), respectively.

The results of this example demonstrated high levels of antibody production in Integra flasks within short periods of time. Cell densities of 1.0×10⁶ and 1.5×10⁷ were examples of optimal densities for producing high levels of antibodies. As shown herein, transiently transfected cells, for example, 293E cells, in membrane-enhancing culturing vessels such as Integra flasks, generated higher total antibody yields over cells cultured in Erlenmeyer flasks, irrespective of antibody productivity levels in shake cultures. Transiently expressing antibodies in membrane-enhancing culturing vessels such as Integra flasks also appeared to better retain antibody stability upon exhaustion of the culture.

As shown by the results obtained herein, total antibody yields from transiently transfected 293E cells are significantly increased when cultured in Integra flasks vs. standard Erlenmeyer flasks. Increasing the number of transfected cells seeded in the Integra flask can substantially decrease the time to reach maximum antibody yield, while decreasing the seeding density allows for multi-mg production of antibodies using a fraction of the cells under normal conditions in an Erlenmeyer flask. Generating transiently expressed antibodies in Integra flasks also better maintains the antibody titer for longer periods of time thus allowing for greater confidence to allow cultures to proceed to extinction without significant loss of antibody. Advantageously, usage of membrane-enhancing culturing vessels, such as Integra flasks, for transient protein production, such as antibody production, allows for increased total yield, faster production by using more cells, and/or conservation of cells by using fewer cells while maintaining productivity comparable to non-membrane culturing vessels, such as Erlenmeyer flasks.

All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 

1. A recombinant expression vector comprising a 3′ UTR of a light chain gene and an Epstein-Barr virus origin of replication (oriP).
 2. The recombinant expression vector of claim 1, wherein the light chain gene is a murine light chain gene.
 3. The recombinant expression vector of claim 2, wherein the 3′ UTR comprises a nucleotide sequence of nucleotides 1062-2560 of SEQ ID NO:
 1. 4. The recombinant expression vector of claim 1, wherein the recombinant expression vector is a recombinant transient expression vector.
 5. The recombinant expression vector of claim 1 further comprising a pUC19 origin of replication, a viral promoter, a 5′ UTR intron, or a combination of any of the foregoing.
 6. The recombinant expression vector of claim 5 further comprising a pUC19 origin of replication, a viral promoter, and a 5′ UTR intron.
 7. The recombinant expression vector of claim 5, wherein the viral promoter is a CMV promoter.
 8. The recombinant expression vector of claim 5, wherein the 5′ UTR intron comprises nucleotides 888-974 of SEQ ID NO:
 1. 9. The recombinant expression vector of claim 5, wherein the pUC19 origin of replication comprises the nucleotide sequence of nucleotides 4551-5220 of SEQ ID NO:
 1. 10. The recombinant expression vector of claim 1 further comprising an antibody signal sequence.
 11. The recombinant expression vector of claim 1, wherein the recombinant expression vector does not comprise an antibody signal sequence.
 12. The recombinant expression vector of claim 1 further comprising a nucleotide sequence encoding a protein or a functional fragment thereof.
 13. The recombinant expression vector of claim 12, wherein the nucleotide sequence encodes an immunoglobulin chain.
 14. The recombinant expression vector of claim 13, wherein the immunoglobulin chain is selected from the group consisting of a γ₁ heavy chain, a γ₂ heavy chain, a γ₄ heavy chain, a κ light chain, and a λ light chain.
 15. A method of producing a recombinant protein, comprising contacting a cell with the recombinant expression vector of claim 1, whereupon a recombinant protein is produced.
 16. A host cell comprising the recombinant expression vector of claim
 1. 17. The host cell of claim 16, wherein the host cell is a mammalian cell.
 18. The host cell of claim 17, wherein the mammalian cell is a human cell.
 19. The host cell of claim 18, wherein the human cell is a human embryonic kidney cell.
 20. The host cell of claim 19, wherein the human embryonic kidney cell expresses an Epstein Barr virus nuclear antigen-1 (EBNA-1) protein.
 21. The host cell of claim 20, wherein the host cell is a 293E cell.
 22. A method of producing a recombinant protein, comprising culturing the host cell of claim 16, whereupon a recombinant protein is produced.
 23. A method of producing a recombinant protein, the method comprising contacting cells in a medium with a first vector and a second vector, wherein the first vector encodes a first polypeptide chain and the second vector encodes a second polypeptide chain, wherein the first polypeptide chain is different from the second polypeptide chain, and wherein the second vector is present in the medium in an amount which is about 1.5 to 2.5 times as much as the amount of the first vector, whereupon a recombinant heterodimeric or heteromultimeric protein is produced.
 24. The method of claim 23, wherein the second vector is present in the medium in an amount which is about 1.75 to 2.25 times as much as the amount of the first vector.
 25. The method of claim 24, wherein the second vector is present in the medium in an amount which is twice as much as the amount of the first vector.
 26. The method of claim 23, wherein the recombinant protein is a heterotetrameric protein.
 27. The method of claim 26, wherein the heterotetrameric protein is an immunoglobulin.
 28. The method of claim 27, wherein the first vector encodes a heavy chain of an immunoglobulin, or a functional fragment thereof, and the second vector encodes a light chain of an immunoglobulin, or a functional fragment thereof.
 29. The method of claim 28, wherein the heavy chain is a human γ₁ heavy chain, human γ₂ heavy chain, or a human γ₄ heavy chain.
 30. The method of claim 28, wherein the light chain is a human κ light chain or a λ light chain.
 31. The method of claim 23, wherein each of the first vector and the second vector is a recombinant transient expression vector.
 32. The method of claim 23, wherein each of the first vector and the second vector comprises a 3′ untranslated region (UTR) of a light chain gene and an oriP.
 33. The method of claim 23, wherein each of the first vector and the second vector comprises a viral promoter, a pUC19 origin of replication, a 5′ UTR intron, or a combination of any of the foregoing.
 34. The method of claim 33, wherein the viral promoter is a CMV promoter.
 35. The method of claim 33, wherein the 5′ UTR intron comprises a nucleotides 888-974 of SEQ ID NO:
 1. 36. The method of claim 23, wherein each of the first vector and second vector comprises an antibody signal sequence.
 37. The method of claim 23, wherein the cells are contacted with the first vector and second vector simultaneously.
 38. The method of claim 23, wherein the cells are contacted with the first vector and second vector in the presence of a cationic polymer.
 39. The method of claim 38, wherein the cationic polymer is polyethyleneimine (PEI).
 40. The method of claim 39, wherein the PEI is a linear PEI.
 41. The method of claim 40, wherein the linear PEI is present in an amount that is about 1.5 to 4.5 times the amount of the first vector and second vector.
 42. The method of claim 41, wherein the linear PEI is present in an amount that is about 2.5 to 3.5 times the amount of the first vector and second vector.
 43. The method of claim 42, wherein the linear PEI is present in an amount that is twice the amount of the first vector and second vector.
 44. The method of claim 23, wherein the cells are mammalian cells.
 45. The method of claim 44, wherein the mammalian cells are human cells.
 46. The method of claim 45, wherein the cells are human embryonic kidney cells.
 47. The method of claim 46, wherein the human embryonic kidney cells express Epstein-Barr virus nuclear antigen-1 protein (EBNA-1).
 48. The method of claim 47, wherein the cells are 293E cells.
 49. The method of claim 23 further comprising isolating the cells from the medium and culturing the cells in a second medium in a membrane-enhanced culturing vessel, wherein the second medium is different from the medium.
 50. The method of claim 49, wherein the second medium is a serum-free cell culture medium.
 51. The method of claim 50, wherein the second medium is IS293™ medium.
 52. The method of claim 49, wherein the membrane-enhanced culturing vessel is an Integra CL1000.
 53. The method of the claim 49 further comprising purifying the recombinant protein from the second medium.
 54. The method of claim 53, wherein the purifying comprises centrifuging the second medium through a column comprising Protein A.
 55. The method of claim 53, wherein the purifying occurs after 3 days of culturing the cells in the second medium.
 56. The method of claim 53, wherein the purifying occurs after 7 days of culturing the cells in the second medium.
 57. The method of claim 55, wherein at least 300 μg/ml recombinant protein is produced in the second medium.
 58. The method of claim 57, wherein at least 500 μg/ml recombinant protein is produced in the second medium.
 59. The method of claim 58, wherein at least 700 μg/ml recombinant protein is produced in the second medium.
 60. The method of claim 49, wherein the culturing comprises seeding cells in the second medium at a cell density between about 1.0×10⁶ and 2.0×10⁷ cells/ml.
 61. The method of claim 60, wherein the cell density is about 3.0×10⁶ to about 1.0×10⁷ cells/ml.
 62. A method of producing a recombinant protein comprising culturing cells, which have been contacted with a recombinant transient expression vector encoding a recombinant protein, in a medium in a membrane-enhanced culturing vessel or in a Fembach flask, whereupon the recombinant protein is produced.
 63. The method of claim 62, wherein the medium is a serum-free cell culture medium.
 64. The method of claim 63, wherein the serum-free medium is IS293™ medium.
 65. The method of claim 62, wherein the membrane-enhanced culturing vessel is an Integra CL1000.
 66. The method of claim 62, wherein the method further comprises purifying the recombinant protein from the medium.
 67. The method of claim 66, wherein the purifying comprises centrifuging the medium through a column comprising Protein A.
 68. The method of claim 66, wherein the purifying occurs after 3 days of culturing the cells in the medium.
 69. The method of claim 66, wherein the purifying occurs after 7 days of culturing the cells in the medium.
 70. The method of claim 68, wherein at least 300 μg/ml recombinant protein is produced in the medium.
 71. The method of claim 70, wherein at least 500 μg/ml recombinant protein is produced in the medium.
 72. The method of claim 71, wherein at least 700 μg/ml recombinant protein is produced in the medium.
 73. The method of claim 62, wherein the culturing comprises seeding cells in the medium at a cell density between about 1.0×10⁶ and 2.0×10⁷ cells/ml.
 74. The method of claim 73, wherein the cell density is 3.0×10⁶ to about 1.0×10⁷ cells/ml.
 75. The method of claim 62, wherein the recombinant transient expression vector comprises a 3′ untranslated region (UTR) of a light chain gene and an oriP.
 76. The method of claim 62, wherein the recombinant transient expression vector comprises a pUC19 origin of replication, a viral promoter, a 5′ UTR intron, or a combination of any of the foregoing.
 77. The method of claim 76, wherein the viral promoter is a CMV promoter.
 78. The method of claim 76, wherein the 5′ UTR intron comprises nucleotides 888-974 of SEQ ID NO:
 1. 79. The method of claim 62, wherein the cells have been contacted with a recombinant transient expression vector in the presence of a cationic polymer.
 80. The method of claim 79, wherein the cationic polymer is a polyethyleneimine (PEI).
 81. The method of claim 80, wherein the PEI is a linear PEI.
 82. The method of claim 81, wherein the PEI is present in an amount that is about 1.5 to 4.5 times the amount of the recombinant transient expression vector.
 83. The method of claim 82, wherein the PEI is present in an amount that is about 2.5 to 3.5 times the amount of the recombinant transient expression vector.
 84. The method of claim 83, wherein the PEI is present in an amount that is twice the amount of the recombinant transient expression vector.
 85. The method of claim 62, wherein the cells are mammalian cells.
 86. The method of claim 85, wherein the mammalian cells are human cells.
 87. The method of claim 86, wherein the human cells are human embryonic kidney cells.
 88. The method of claim 87, wherein the human embryonic kidney cells express Epstein-Barr virus nuclear antigen-1 protein (EBNA-1).
 89. The method of claim 88, wherein the cells are 293E cells.
 90. The method of claim 62, wherein the protein is an immunoglobulin chain or a functional fragment thereof.
 91. The method of claim 90, wherein the immunoglobulin chain is a heavy chain or a light chain.
 92. The method of claim 91, wherein the immunoglobulin chain is a heavy chain, and the heavy chain is a human γ₁ heavy chain, human γ₂ heavy chain, or a human γ₄ heavy chain.
 93. The method of claim 91, wherein the immunoglobulin chain is a light chain, and the light chain is a human κ light chain or a λ light chain. 